1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission for a vehicle such as an automobile, and more particularly, to a hydraulic control device for such an automatic transmission having two parallelly arranged input clutches which are selectively engaged or disengaged in providing a plurality of speed stages.
2. Description of the Related Art
In Japanese Patent Application No. 62-195471 filed on Aug. 5, 1987 and laid open to public on Feb. 13, 1989, two of the inventors of the present invention have proposed an automatic transmission for a vehicle such as an automobile in which a speed stage shifting mechanism includes two parallelly arranged input clutches and is adapted so that a first one of the two input clutches is engaged when it provides a first speed stage, a second speed stage and a third speed stage, whereas a second one of the two input clutches is engaged when it provides the third speed stage and a fourth speed stage. In such a speed stage shifting mechanism the engagement of said first input clutch (in fact clutch C.sub.1 described hereinunder) is not essential, however, for setting up said third speed stage, if no engine brake availability is required, by a provision of a one way clutch in parellel with said first input clutch, said one way clutch being adapted to be automatically engaged when the automatic transmission operates in a normal engine driving condition. For setting up said second speed stage in which said second input clutch is disengaged, the engagement of said first input clutch is essential.
In such an automatic transmission, assuming that the vehicle is running with the automatic transmission being set up in the fourth speed stage, there are two possibilites for the transmission being shifted down from the fourth speed stage to the third speed stage. In a first possibility, the automatic transmission is shifted down from the fourth speed stage to the third speed stage by a stepping down of the accelerator pedal, that is, the so-called kickdown, and in a second possibility the transmission is shifted down from the fourth speed stage to the third speed stage by the prohibition of the overdrive stage, provided that the fourth speed stage is an overdrive stage as mostly so in the modern automobiles and there is provided an overdrive selection means such as an overdrive cutoff button.
When the transmission is shifted down from the fourth speed stage to the third speed stage in the above-mentioned type automatic transmission, it is desired that, when the downshifting is due to the kickdown, said first input clutch is engaged relatively slowly as compared in the downshifting due to the overdrive prohibition, or in other words, when the downshifting is due to the overdrive prohibition, said first clutch is engaged relatively quickly as compared in the downshifting due to the kickdown, because, when the automatic transmission is shifted down from the fourth speed stage to the third speed stage by the kickdown, after a reaction brake (in fact brake B.sub.1 described hereinunder) was disengaged, engine rotational speed rises and the above-mentioned one way clutch (in fact one way clutch F.sub.1 described hereinunder) is automatically put into engagement, and therefore said first clutch may be slowly engaged or rather should be slowly engaged not to cause an abrupt engine braking, while in the downshifting due to the overdrive prohibition, since the accelerator pedal will not be stepped on, engine rotational speed will not rise so that said one way clutch is swiftly engaged, and therefore, if the engagement of the first clutch is delayed, the vehicle will run in an inertial condition as if it is running with the manual shift lever set to the neutral position. Such an inertial running condition, when it occurs without a positive instruction by the drive, will give the driver a feeling of danger of loss of engine braking.
Further, when the transmission is shifted down from the fourth speed stage to the second speed stage by a deeper depression of the accelerator pedal, it is now essential that said first clutch is engaged to operate the transmission since said second clutch is now disengaged, and in this case said first clutch should be engaged quickly.
Thus, it is noted that in the automatic transmission of the above-mentioned type the speed of engagement of said first clutch should be variably controlled according to the stepping on condition of the accelerator pedal or, more fundamentally, the output power of the engine.
In order to control the engagement speed of said first input clutch in accordance with the stepping on condition of the accelerator pedal, it is considered to operate a switching over valve in the hydraulic control system based upon the so-called throttled pressure which is well known in the hydraulic control system of the automatic transmission for a vehicle such as an automobile as a hydraulic pressure generated by a pressure modulating valve which modulates a source hydraulic pressure generally available as the so-called line pressure in the hydraulic control system according to a mechanical displacement of a valve spool element representing the stepping on movement of the accelerator pedal. However, the throttle pressure is generally designed to correspond to engine output power in as a wide range thereof as possible, and therefore the rate of increase or decrease of the throttle pressure relative to increase or decrease of engine output power is generally too small in a region of small engine output power. In other words, as viewed in a diagram showing the pressure level of the throttle pressure along an ordinate relative to throttle opening along an abscissa, the inclination of a curve of the throttle pressure is too small to faithfully simulate engine output power in a region of low throttle opening.